ZFIN ID: ZDB-PUB-961014-587
Reticular interneurons with T-shaped axons in embryos of the zebrafish
Kimmel, C.B., Metcalfe, W.K., and Schabtach, E.
A class of interneuron was identified in embryos and young larvae of Brachydanio rerio which may function during fast-start behaviors initiated by the Mauthner neuron. The cells were labeled with HRP applied to lesions of their axons within the brain. The somata are located laterally in the reticular formation of the 'caudal hindbrain. The proximal segments of their axons project
medially to cross the midline, and bifurcate to form rostral and caudal segments which course within the medial longitudinal fasciculus. The T-shaped axons of these neurons distinguish them from previously described interneurons in the zebrafish (Kimmel, C. B. et al., J. Compo Neur., 205: 112, 1982).
We have observed 8-10 of these neurons, present in a rostro-caudal series, on each side of the brain. However, in most experiments fewer cells are labeled. Cells located caudally in the set are fusiform in shape and possess thin axons. Two very prominent cells are located rostrally in the set on each side. Their somata are round, their dendrites short, and their axons are of large diameter, second in size only to the Mauthner axon. They are identifiable by the second day after fertilization. The rostral most cells receive axo-axonal synapses from each Mauthner axon, as well as from other unidentified axons in the medial longitudinal fasciculus. Under EM these contacts are seen to be Type I chemical synapses. Both rostral and caudal segments give rise to laterally directed collaterals, which thus project exclusively contralateral to the somata. A large collateral of the rostral segment projects to the trigeminal motor nucleus, and the rostral segment itself forms a distinctive hook-shaped ending within the oculomotor nucleus. The caudal segment forms terminal arbors near the bulbo-spinal junction. Motoneurons at this location innervate musculature of the pectoral fins.
A number of these morphological features are similar to those of the previously described "giant fiber" cells of the adult hatchet fish (Model, P. G. et al., Brain Res., 45: 288, 1972), which receive input from the Mauthner neurons and mediate bilateral responses of the pectoral fins. We propose that the rostral interneurons described here are homologous. It is known also that cranial musculature is bilaterally excited during Mauthner cell initiated behavior (Diamond, J., in Fish Physiology, 5: 265, 1971}. Such responses may be mediated by the rostral-axonal segments of these interneurons. (Supported by NIH grant NS 176963.)
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